This invention relates to electrophotography in which a semiconducting or insulating magnetic toner is used and to developing an electrostatic latent image according to a magnetic brush method. The latent image thus developed is transferred onto a transferring member such as an ordinary sheet and is then subjected to fixing to obtain a copy.
In electrophotography, an electrostatic latent image is formed on the photo-conductive surface of a photo-sensitive material and is then developed to obtain a toner image. The toner image is either subjected directly to fixing, or after it is transferred onto a transferring member, it is subjected to fixing to obtain a copy.
The magnetic brush development method is extensively employed to develop an electrostatic latent image. Heretofore, a two-component system developer, which is a mixed powder comprising a non-magnetic toner and a carrier such as iron powder, has been employed as the developer in this method. In magnetic brush development methods using the two-component system developer, the toner is charged to have a predetermined polarity by the friction caused between the toner and the carrier. As a result only the toner is allowed to stick to the surface where the electrostatic latent image is formed. This method is advantageous in that the latent image can be readily transferred.
However, this method is still disadvantageous in the following points: It is necessary to provide a mixing means to charge the toner by the utilization of the friction caused between the toner and the carrier. Furthermore, it is necessary to use a toner supplying means in order to maintain a uniform density of toner because only the toner is consumed during development. Therefore, the size of the developing device is necessarily large. If the developer is used for extended periods of time, the carrier deteriorates, and therefore it is necessary to replace the carrier.
In order to eliminate the above-described difficulties, a one-component system non-chargeable magnetic toner comprised essentially of resin and magnetic powder is employed as the developer. A development method using this magnetic toner is disclosed in the specification of Japanese Patent Application Laid-Open No. 4532/1974. In this development method, an electrically conductive magnetic toner having an electrical resistance of the order of 10.sup.4 -10.sup.11 .OMEGA.cm under the application of an electric field of DC 100 V/cm is used as the magnetic toner.
An electrically conductive sleeve is employed to carry the magnetic toner and the rear surface of an electrostatic latent image carrier is electrically connected to the electrically conductive sleeve so that an electrically conductive path is formed between the surface of the electrostatic latent image carrier and the sleeve through the conductive magnetic toner layer. Hence, the charges which are induced on the conductive sleeve by the charges of the electrostatic latent image and are opposite in polarity to those of the electrostatic latent image are collected into the top of the toner brush. Also, the conductive sleeve and the electrostatic latent image carrier are moved relatively so that a coulomb force created by both the charges in the top of the toner brush and the charges of the electrostatic latent image overcome the magnetic attraction force of a permanent magnet roll provided in the conductive sleeve. This permits the toner to selectively deposit on the surface of the electrostatic latent image.
This method may be applied to a so-called coated paper copying or CPC system in which a toner image obtained through development is fixed to directly obtain a copy because the toner is electrically conductive. However, in applying the method to the CPC method, it is necessary to provide a means of forming an electrically conductive path at all times, for instance, by grounding a rotary member, and therefore the construction of the developing device becomes necessarily intricate. In addition, electrical paths are formed between toners, and therefore if the toners are strongly charged in one polarity, a fog-like condition is created.
In the method in which the electrically conductive toner is used and development is effectuated with the electrically conductive sleeve, the electrical resistance of the toner is relatively low, and therefore transferring the toner image onto the transferring member involves the problem where the toner is scattered or the transferring electric field is weakened because of leakage. Accordingly, this method cannot be applied to a so-called plain paper copying or PPC system in which a copy is obtained through development, transferring and fixing processes; that is, as the toner's electrical resistance is low, the transferring efficiency is low. Therefore, it is impossible to provide a copy having a desired density. Furthermore, in the case where the above-described developing method is applied to the PPC developing system, with a semiconducting or insulating non-chargeable type magnetic toner poor in development characteristic and high in resistance, the amount of toner stuck to the photo-sensitive material is decreased, and accordingly an excellent copy cannot be obtained.
Another example of the magnetic brush development method for the CPC system is described in the specification of Patent No. 4,126,100 to Nishihama. In this method, in moving the non-magnetic sleeve and the photo-sensitive material at a substantially equal speed, the gap between the non-magnetic sleeve and the photo-sensitive material is adjusted so that a toner pool is formed upstream of the development section, and the gap is made smaller than the thickness of toner to further create a physical disturbance to the toner in the toner pool. In this conventional method, the magnet roll provided in the sleeve is rotated in a direction opposite to the direction of rotation of the sleeve in order to magnetically disturb the toner in the toner pool. If the magnet roll and the sleeve are rotated in opposite directions, then the apparent speed of movement of the toner is increased, so that an excessively great force is applied to the toner. As a result blocks of toner are liable to be formed because the toner is essentially made of soft resin and is weak in mechanical strength.
Furthermore, both a rotation force due to the rotating magnetic force of the magnet roll and a revolution force due to the movement of the sleeve are applied to the toner, so that the toner is carried at high speed and the toner brush strongly rubs the photo-sensitive surface. As a result, the photo-sensitive material is deteriorated. Thus, this method is not practical. In the case where the method is applied to the PPC development system, the development characteristic is similarly low as in the method described in the above-described U.S. patent, and the density and tone of a copy obtained with the PPC magnetic toner are unsatisfactory.
In order to apply the magnetic toner to the PPC development system, a method has been proposed in which the development is carried out by using an insulating magnetic toner whose electrical resistance is increased, and then the transferring process is performed. For instance, Japanese Patent Application Laid-Open No. 92137/1975 discloses a method in which a magnetic toner having an electrical resistance of 10.sup.6 -10.sup.16 .OMEGA.cm is brought into contact with an electrostatic latent image for polarization. However, this conventional method involves particularly difficult practical problems. For instance, if a magnetic toner having a low electrical resistance of the order of 10.sup.6 .OMEGA.cm is transferred, then the toner image is greatly deformed because of the low electrical resistance. Accordingly, magnetic toner cannot be used.
If a magnetic toner having a high electrical resistance of 10.sup.12 .OMEGA.cm or higher is used, it is impossible to carry out development in the conventional developing method because of the excessively high electrical resistance. In addition, it is apparent that it is impossible to obtain satisfactory development merely by bringing the toner closer to the electrostatic latent image, or with a weak polarization force, as in the method. It is necessary to use a semiconducting or insulating magnetic toner having an electrical resistance of 10.sup.8 .OMEGA.cm or higher for transferring the toner image. In order to correctly develop the latent image on the photo-sensitive material by sticking such a toner thereto, it is necessary to provide a mechanism for generating a force to stick the toner thereto.
Another developing method applicable to the PPC system has been disclosed by Japanese Patent Laid-Open No. 129639/1978. In this conventional method, the sleeve is rotated in such a manner that the difference in speed between the sleeve and the photo-sensitive material is within a predetermined range, to eliminate the occurrence of those image, edge effects and fog. In this method, only the sleeve is rotated. Accordingly, it is difficult to uniformly control the toner at the doctor section; that is, blocks of toner or dust is liable to be caught by the doctor section. As a result, strips due to insufficient development may appear by the shortage of toner on the sleeve. In addition, since the magnetic brush is dynamically struck against the position where the toner is brought into contact with the development surface, the abrasion force is increased. This is effective for cleaning, but lowers the development characteristic. Furthermore, necessarily it is difficult to allow the toner to stick to the half tone region where the potential of the electrostatic latent image is low, and the resultant image is hard and is not a correct copy of the original.
In a developing device according to the sleeve rotation system as disclosed by the above-described Japanese Patent Application Laid-Open No. 129639/1978, the positions of the magnetic poles are fixed, and therefore if the positional relation of the magnetic poles is even slightly changed, the quality of the resultant image is greatly changed. Thus, the developing device suffers from the necessity of finely adjusting the positions of the magnetic poles. The development gap and the doctor gap are defined in the developing device, so that the toner strongly abrades the photo-sensitive material. Furthermore, the development characteristic, that is, the toner adhesion characteristic to the photo-sensitive material is lowered when the toner movement speed is high as in the sleeve rotation system (described later). Thus, the developing device is not applicable to a PPC development system using a high resistance toner.
As is apparent from the above description, it is impossible to provide an excellent image by the sleeve rotation system. In order to improve the sleeve rotation system, a method has been proposed by Nelson in U.S. Pat. No. 4,121,931. In this conventional method, the toner movement speed is increased to improve the development characteristic of the insulating magnetic toner. Especially, even with an insulating magnetic toner, the toner is forcibly charged by bringing it in contact with the electrodes and is then carried to the latent image surface at a speed higher than 10 cm/sec. This obtains a development characteristic equivalent to that of the case of the electrically conductive toner. However, this conventional method still has the above-described drawbacks accompanying the sleeve rotation system, and additionally a drawback is caused by the higher toner movement speed with respect to the photo-sensitive material.
The specification of Nelson describes that the magnet roll may be rotated instead of the sleeve, or both the magnet roll and the sleeve may be rotated. In any case, the toner movement speed is high, and therefore the above-described problem cannot be solved.
The toner described in the above mentioned specification should have a conductivity 10.sup.-12 .OMEGA./cm or less under the application of an electric field of DC 10,000 .OMEGA./cm to improve the transferring characteristic. The conventional method of the above-described specification is based on the technical concept where, as described later, a magnetic toner charged before the development is carried at a high speed similar to the two-component system toner, to increase the developing current. This increases the amount of toner adhesion. However, since development is greatly affected by the kinds of photo-sensitive materials, the kinds of toners, the magnetic force of a magnet roll, or the like, it cannot be considered that this conventional method is a practical magnetic toner development method.
The mechanism of development will now be described in more detail. As is described in the specification of the aforementioned Nelson patent, as the toner is brought into contact with the latent image surface at higher speed, that is, the speed of the toner leaving the latent image surface is increased, the cleaning effect of the magnetic brush is increased. Therefore, although the toner charged by the electrodes is stuck to the latent image surface with the aid of the coulomb force, it is recovered from the latent image surface, thus lowering the development efficiency.
This drawback may be eliminated by increasing for example the rotational rate of the sleeve. However, in this case, the amount of charge of the toner becomes short, and a cleaning effect is provided by a strong abrasion force which is generated when the magnetic brush passes over the development surface. As a result the development efficiency is further lowered. Thus, it is impossible to obtain excellent development merely by controlling the toner movement speed and the amount of charges of the toner.
The specification of the aforementioned Patent to Nelson describes an embodiment where the magnet roll and the sleeve are rotated in the opposite directions at 1500 r.p.m. and 300-400 r.p.m., respectively. However, in this method, the toner movement speed is very high, and therefore the above-described difficulty accompanying the increase of the toner movement speed to 10 cm/sec or higher cannot be eliminated. The toner movement speed is the sum of the sleeve speed and the magnet sleeve; that is, it is the sum of the speed of rotation of the toner and the speed of revolution are the same due to the peripheral velocity of the sleeve. Thus, the toner movement speed is considerably increased. Furthermore, even if the high resistance magnetic toner for the PPC system can develop the latent image on the photo-sensitive material, although the developed image is insufficient, nothing is taken into account for transferring the developed image onto a transferring member in the conventional method. Accordingly, it is obvious that an excellent copy cannot be obtained simply by the above-described developing method in view of the copying process.
In order to improve the development characteristic in the use of the high resistance magnetic toner, a developing method has been disclosed by Japanese Patent Application Laid-Open No. 117432/1975. In this conventional developing method, a toner is forcibly charged by using a corona charger, or electrodes such as doctor blades. Thus, the developing method is similar to the method of using the two-component system toner in which the toner is charged before development. It cannot be considered possible to put the method in practical use because the method involves difficulties such as the occurrence of leakage in the process of up to development and non-uniform charging due to dirty conditions of the charger.
As is apparent from the above description, the insulating magnetic toner lowers the development characteristic, and the semiconducting or conducting magnetic toner lowers the transferring characteristic. It is generally considered impossible to improve the development characteristic according to the conventional methods. Accordingly, a variety of magnetic toner PPC copying machines recently developed use toners having a relatively low electrical resistance of the order of 10.sup.7 -10.sup.9 .OMEGA.cm to achieve an excellent development characteristic. In this connection, in order to improve the transferring characteristic, a transferring sheet whose saturation potential is not significantly decreased under an environmental condition of high humidity as disclosed by Japanese Patent Application Laid-Open No. 38752/1978 has been proposed. Also, a transferring sheet having a high volumetric resistivity at a high humidity as disclosed by Japanese Patent Application Laid-Open No. 131044/1978 has been proposed.
However, as a result of tests of the former transferring sheet, it has been found that with the transferring sheet whose saturation potential is not significantly decreased at a high humidity, it is impossible to satisfactorily transfer magnetic toner thereof. This is especially true in the case where the saturation potential is excessively high: it is difficult to peel the transferring sheet off the photo-sensitive material after the transferring operation, and the image is deformed by electrical discharge. If in contrast, the saturation potential is excessively low, the toner image cannot be transferred satisfactorily due to the leakage of transferring charges. In the case where a transferring sheet is used which has no problem for transferring the two-component system toner, with the one-component system toner the above-described deformation of the image caused in the transferring operation can be avoided. Therefore, it is considered essential to define some characteristics other than the saturation potential.
In the aforementioned Japanese Patent Application Laid-Open No. 131044/1978, the volumetric resistivity of the transferring sheet is defined. However, as a result of a detailed investigation, the inventors have confirmed that if the volumetric resistivity is 10.sup.10 .OMEGA.cm or higher, the resultant transferring characteristic is insufficient for the transferring sheet of the one-component system magnetic toner.